1. Field of Invention
This invention pertains to digital modems and more particularly to a digital modem using QAM type modulation in which an automatic gain control circuit is used with a dynamically changing gain signal dependent on the extreme inner or outer points of the QAM signal constellation.
2. DESCRIPTION OF THE PRIOR ART
While in analog communication systems errors caused by various interferences due to external causes or channel distortions can be usually tolerated, in digital data communications such errors can have grave consequences and therefore great and painstaking efforts have to be made to minimize them. For example many modems are provided with automatic gain control (AGC) circuits which monitor the amplitude levels of the received signals and generate an output to the demodulator section of a receiver. The AGC gain is dynamically adjusted by using the RMS value of the incoming signals to try to maintain said output within a preselected amplitude range. However it was found that for relatively fast excursions of the received signals, these AGC circuits are ineffective even with dynamic equalization.
The shortcomings of the prior art are especially acute in quadrature amplitude modulation systems having relatively large and/or dense signal constellations used for data rates in excess of 14,400 bits per second. The averaging AGC and adaptive equalizers are slow and cannot react fast enough to large gain hits of 2-3 db because the outer points of the signal constellation are shifted towards the columns of the inner points and interpreted incorrectly as error signals. The problem is aggravated in the systems using forward error correction schemes due to the inherent delay in such systems between the received QAM signals and demodulated data signals.